If the output spline rotates constantly even when you aren't touching the stick then the problem is that the servo is not getting the feedback about where the spline position actually is.
Basically this means that there is somethng wrong with the potentiometer or it's attachment to the bottom of the main spline gear.
I think ashleyb4 kind of described this.
To check this you need to remove the spline gear from the servo and check that the bottom of it is not damaged and slipping on the pot.
If it is not slipping then the pot or part of the electrics is shot and that is not really user-serviceable.
If the servo stays in neutral but rotates constantly when you move the stick, then the end stops are damaged, this is a raised part on the spline gear combined with a moulding on the servo case. This is what Col was referring to.
HTH
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